Various studies have been conducted as to a sol liquid for silanol porous body that can form a structure with void spaces (hereinafter, also referred to as a “void-provided structure”) using a silica compound material (silicon compound material) as a raw material. In such studies, the following point is in common. That is, after gelation of a silica compound, a sol liquid in which the gelled silica compound is pulverized is prepared, and a base is coated with the sol liquid to form a void-provided structure. However, there is a problem that an attempt of achieving a higher porosity decreases the film strength of the silanol porous body significantly, which makes it difficult to industrially obtain a silanol porous body in a simple manner. Regarding an example of a member, which achieves a high porosity and a high strength, there are methods of applying the member to an antireflection layer of a lens (for example, see Patent Documents 1 to 4). In each of these methods, a layer with void spaces (hereinafter, also referred to as a “void-provided layer”) is formed on a lens and then baked at a high temperature of 150° C. or more for a long time. These methods, however, have the following problem. Since the gel obtained by using tetraethoxysilane (TEOS) as a raw material is inferior in flexibility, the porous body cannot be formed on a soft base. On the other hand, there is an example of application of a void-provided layer, without carrying out baking treatment (for example, see Non-Patent Document 1). This method, however, has the following problem. Since a number of residual silanol groups remain in the silanol pulverized sol and the formed void-provided layer is not subjected to the baking treatment, the porous body to be obtained is inferior in film strength and does not achieve an impact resistance.
For solving such problems, there are attempts to develop a film that can be a substitute for an air layer which is a void space between components.